Oled with protective bi-layer electrode

ABSTRACT

An OLED device, comprising: a substrate; a first electrode formed over the substrate; one or more organic layers formed over the first electrode, at least one organic layer being a light-emitting layer; and a bi-layer electrode comprising a first transparent conductive protective hermetic layer formed over the one or more organic layers, and a second transparent conductive non-hermetic layer formed over the first transparent conductive protective hermetic layer.

FIELD OF THE INVENTION

The present invention relates to organic light-emitting diode (OLED)devices, and more particularly, to OLED device structures for improvinglight output and lifetime formed by the deposition of thin-filmmaterials, employing a process for forming a transparent conductiveprotective layer by vapor deposition.

BACKGROUND OF THE INVENTION

Organic light-emitting diodes (OLEDs) are a promising technology forflat-panel displays and area illumination lamps. The technology reliesupon thin-film layers of organic materials coated upon a substrate. OLEDdevices generally can have two formats known as small-molecule devicessuch as disclosed in U.S. Pat. No. 4,476,292 and polymer OLED devicessuch as disclosed in U.S. Pat. No. 5,247,190. Either type of OLED devicemay include, in sequence, an anode, an organic EL element, and acathode. The organic EL element disposed between the anode and thecathode commonly includes an organic hole-transporting layer (HTL), anemissive layer (EL) and an organic electron-transporting layer (ETL).Holes and electrons recombine and emit light in the EL layer. Tang etal. (Appl. Phys. Lett., 51, 913 (1987), Journal of Applied Physics, 65,3610 (1989), and U.S. Pat. No. 4,769,292) demonstrated highly efficientOLEDs using such a layer structure. Since then, numerous OLEDs withalternative layer structures, including polymeric materials, have beendisclosed and device performance has been improved. However, thematerials comprising the organic EL element are sensitive and, inparticular, are easily destroyed by moisture and high temperatures (forexample greater than 140 degrees C.).

It has been found that one of the key factors that limits the efficiencyof OLED devices is the inefficiency in extracting the photons generatedby the electron-hole recombination out of the OLED devices. Due to therelatively high optical indices of the organic and transparent electrodematerials used, most of the photons generated by the recombinationprocess are actually trapped in the devices due to total internalreflection. These trapped photons never leave the OLED devices and makeno contribution to the light output from these devices. Because light isemitted in all directions from the internal layers of the OLED, some ofthe light is emitted directly from the device, and some is emitted intothe device and is either reflected back out or is absorbed, and some ofthe light is emitted laterally and trapped and absorbed by the variouslayers comprising the device. In general, up to 80% of the light may belost in this manner. Moreover, the organic and electrode materialsabsorb light, further reducing the device efficiency.

A typical OLED device uses a glass substrate, a transparent conductinganode such as indium-tin-oxide (ITO), a stack of organic layers, and areflective cathode layer. Light generated from such a device may beemitted through the glass substrate. This is commonly referred to as abottom-emitting device. Alternatively, a device can include a substrate,a reflective anode, a stack of organic layers, and a top transparentcathode layer. Light generated from such an alternative device may beemitted through the top transparent electrode. This is commonly referredto as a top-emitting device. In these typical devices, the index of theITO layer, the organic layers, and the glass is about 1.8-2.0, 1.7, and1.5 respectively. It has been estimated that nearly 60% of the generatedlight is trapped by internal reflection in the ITO/organic EL element,20% is trapped in the glass substrate, and only about 20% of thegenerated light is actually emitted from the device and performs usefulfunctions.

A variety of techniques have been proposed to improve the out-couplingof light from thin-film light emitting devices. One such technique,taught in US 2006/0186802 entitled “OLED Device having Improved LightOutput” by Cok et al, which is hereby incorporated in its entirety byreference, describes the use of scattering layers formed over thetransparent electrode of a top-emitter OLED device. It also teaches theuse of very thin layers of transparent encapsulating materials depositedon the electrode to protect the electrode from the scattering layerdeposition. Preferably, the layers of transparent encapsulating materialhave a refractive index comparable to the refractive index range of thetransparent electrode and organic layers, or is very thin (e.g., lessthan about 0.2 micron) so that wave guided light in the transparentelectrode and organic layers will pass through the layers of transparentencapsulating material and be scattered by the scattering layer.

To obtain a sufficiently thick layer having reasonable conductivity in areasonable deposition timeframe, transparent conductive electrodestypically employ sputter-deposited conductive metal oxides such asindium tin oxide. Such electrodes still may suffer from a variety ofhandicaps, principally inadequate conductivity and, in somecircumstances, transparency. In top-emitting device structures, suchsputter-deposited layers may also physically damage the underlyingorganic material layers. Barrier layers providing sputter-depositionprotection have been suggested, such as described in WO 97/42666 andU.S. Pat. No. 6,488,555. However, the described barrier layers andsputter deposited electrode layers typically are not sufficientlyimpermeable to environmental contaminants when employed as thetransparent top electrode in a top-emitting device, necessitating theuse of additional encapsulating overcoat layers or sealed transparentglass covers, thereby exacerbating problems with light trapping and/orincreased costs for such devices.

It is well known that OLED materials are subject to degradation in thepresence of environmental contaminants, in particular moisture. Organiclight-emitting diode (OLED) display devices typically require humiditylevels below about 1000 parts per million (ppm) to prevent prematuredegradation of device performance within a specified operating and/orstorage life of the device. Control of the environment to this range ofhumidity levels within a packaged device is typically achieved byencapsulating the device with an encapsulating layer and/or by sealingthe device, and/or providing a desiccant within a cover. Desiccants suchas, for example, metal oxides, alkaline earth metal oxides, sulfates,metal halides, and perchlorates are used to maintain the humidity levelbelow the above level. See for example U.S. Pat. No. 6,226,890 B1 issuedMay 8, 2001 to Boroson et al. describing desiccant materials formoisture-sensitive electronic devices. Such desiccating materials aretypically located around the periphery of an OLED device or over theOLED device itself.

In alternative approaches, an OLED device is encapsulated using thinmulti-layer coatings of moisture-resistant material. For example, layersof inorganic materials such as metals or metal oxides separated bylayers of an organic polymer may be used. Such coatings have beendescribed in, for example, U.S. Pat. Nos. 6,268,695, 6,413,645 and6,522,067. A deposition apparatus is further described in WO2003090260A2 entitled “Apparatus for Depositing a Multilayer Coating on DiscreteSheets”. WO0182390 entitled “Thin-Film Encapsulation of OrganicLight-Emitting Diode Devices” describes the use of first and secondthin-film encapsulation layers made of different materials wherein oneof the thin-film layers is deposited at 50 nm using atomic layerdeposition (ALD) discussed below. According to this disclosure, aseparate protective layer is also employed, e.g. parylene. Such thinmulti-layer coatings typically attempt to provide a moisture permeationrate of less than 5×10⁻⁶ gm/m²/day to adequately protect the OLEDmaterials. In contrast, typically polymeric materials have a moisturepermeation rate of approximately 0.1 μm/m²/day and cannot adequatelyprotect the OLED materials without additional moisture blocking layers.With the addition of inorganic moisture blocking layers, 0.01 gm/m²/daymay be achieved and it has been reported that the use of relativelythick polymer smoothing layers with inorganic layers may provide theneeded protection. Thick inorganic layers, for example 5 microns or moreof ITO or ZnSe, applied by conventional deposition techniques such assputtering or vacuum evaporation may also provide adequate protection,but thinner conventionally coated layers may only provide protection of0.01 gm/m²/day.

WO2004105149 A1 entitled “Barrier Films for Plastic SubstratesFabricated by Atomic Layer Deposition” published Dec. 2, 2004 describesgas permeation barriers that can be deposited on plastic or glasssubstrates by atomic layer deposition (ALD). Atomic Layer Deposition isalso known as Atomic Layer Epitaxy (ALE) or atomic layer CVD (ALCVD),and reference to ALD herein is intended to refer to all such equivalentprocesses. The use of the ALD coatings can reduce permeation by manyorders of magnitude at thicknesses of tens of nanometers with lowconcentrations of coating defects. These thin coatings preserve theflexibility and transparency of the plastic substrate. Such articles areuseful in container, electrical, and electronic applications. However,such protective layers also cause additional problems with lighttrapping in the layers since they may be of lower index than thelight-emitting organic layers.

Other multi-layer structures are described in the art, for exampleUS20030193286 A1 entitled “Hermetic encapsulation of organic,electro-optical elements” describes a vitreous structure over anelectro-optical element. An encapsulating electrode is described inUS20040070334 entitled “Encapsulated Electrode”. WO2005064993 entitled“Flexible Electroluminescent Devices” describes a multilayer upperelectrode comprising a relatively transparent conductive layer coveredwith an index-matching layer in order to enhance the light output.However, none of these disclosures provide an improved depositionprocess with suitable environmental stability and improved lightemission efficiency.

Composite transparent electrodes comprising a relatively thin metallayer and a relatively thicker transparent conductive metal oxide suchas ITO are also known. In such composite electrodes, the thin metallayer is provided to enhance conductivity of the transparent electrodeand/or provide optical cavity effects. When present, such thin metallayers are typically evaporation or sputter deposited, and do notprovide a hermetic encapsulation. Further, such thin metal layers, evenwhen thin enough to provide at least a minimal transparency, may stillabsorb a significant fraction of light.

Among the techniques widely used for thin-film deposition is ChemicalVapor Deposition (CVD) that uses chemically reactive molecules thatreact in a reaction chamber to deposit a desired film on a substrate.Molecular precursors useful for CVD applications comprise elemental(atomic) constituents of the film to be deposited and typically alsoinclude additional elements. CVD precursors are volatile molecules thatare delivered, in a gaseous phase, to a chamber in order to react at thesubstrate, forming the thin film thereon. The chemical reaction depositsa thin film with a desired film thickness. Common to most CVD techniquesis the need for application of a well-controlled flux of one or moremolecular precursors into the CVD reactor. A substrate is kept at awell-controlled temperature under controlled pressure conditions topromote chemical reaction between these molecular precursors, concurrentwith efficient removal of byproducts. Obtaining optimum CVD performancerequires the ability to achieve and sustain steady-state conditions ofgas flow, temperature, and pressure throughout the process, and theability to minimize or eliminate transients.

Atomic layer deposition (“ALD”) is an alternative film depositiontechnology that can provide improved thickness resolution and conformalcapabilities, compared to its CVD predecessor. In the presentdisclosure, the term “vapor deposition” includes both ALD and CVDmethods. The ALD process segments the conventional thin-film depositionprocess of conventional CVD into single atomic-layer deposition steps.Advantageously, ALD steps are self-terminating and can deposit preciselyone atomic layer when conducted up to or beyond self-terminationexposure times. An atomic layer typically ranges from about 0.1 to about0.5 molecular monolayers, with typical dimensions on the order of nomore than a few Angstroms. In ALD, deposition of an atomic layer is theoutcome of a chemical reaction between a reactive molecular precursorand the substrate. In each separate ALD reaction-deposition step, thenet reaction deposits the desired atomic layer and substantiallyeliminates “extra” atoms originally included in the molecular precursor.In its most pure form, ALD involves the adsorption and reaction of eachof the precursors in the complete absence of the other precursor orprecursors of the reaction. In practice in any process it is difficultto avoid some direct reaction of the different precursors leading to asmall amount of chemical vapor deposition reaction. The goal of anyprocess claiming to perform ALD is to obtain device performance andattributes commensurate with an ALD process while recognizing that asmall amount of CVD reaction can be tolerated.

In ALD applications, typically two molecular precursors are introducedinto the ALD reactor in separate stages. For example, a metal precursormolecule, ML_(x), comprises a metal element, M that is bonded to anatomic or molecular ligand, L. For example, M could be, but would not berestricted to, Al, W, Ta, Si, Zn, etc. The metal precursor reacts withthe substrate, when the substrate surface is prepared to react directlywith the molecular precursor. For example, the substrate surfacetypically is prepared to include hydrogen-containing ligands, AH or thelike, that are reactive with the metal precursor. Sulfur (S), oxygen(O), and Nitrogen (N) are some typical A species. The gaseous precursormolecule effectively reacts with all of the ligands on the substratesurface, resulting in deposition of a single atomic layer of the metal:

substrate-AH+ML _(x)→substrate-AML _(x-1) +HL  (1)

where HL is a reaction by-product. During the reaction, the initialsurface ligands, AH, are consumed, and the surface becomes covered withL ligands, which cannot further react with metal precursor ML_(x).Therefore, the reaction self-terminates when all the initial AH ligandson the surface are replaced with AML_(x-1) species. The reaction stageis typically followed by an inert-gas purge stage that eliminates theexcess metal precursor from the chamber prior to the separateintroduction of the other precursor.

A second molecular precursor then is used to restore the surfacereactivity of the substrate towards the metal precursor. This is done,for example, by removing the L ligands and redepositing AH ligands. Inthis case, the second precursor typically comprises the desired (usuallynonmetallic) element A (i.e., O, N, S), and hydrogen (i.e., H₂O, NH₃,H₂S). The next reaction is as follows:

substrate-A-ML+AH _(Y)→substrate-A-M-AH+HL  (2)

This converts the surface back to its AH-covered state. (Here, for thesake of simplicity, the chemical reactions are not balanced.) Thedesired additional element, A, is incorporated into the film and theundesired ligands, L, are eliminated as volatile by-products. Onceagain, the reaction consumes the reactive sites (this time, the Lterminated sites) and self-terminates when the reactive sites on thesubstrate are entirely depleted. The second molecular precursor then isremoved from the deposition chamber by flowing inert purge-gas in asecond purge stage.

In summary, then, an ALD process requires alternating in sequence theflux of chemicals to the substrate. The representative ALD process, asdiscussed above, is a cycle having four different operational stages:

1. ML_(x) reaction;

2. ML_(x) purge;

3. AH_(y) reaction; and

4. AH_(y) purge, and then back to stage 1.

This repeated sequence of alternating surface reactions andprecursor-removal that restores the substrate surface to its initialreactive state, with intervening purge operations, is a typical ALDdeposition cycle. A key feature of ALD operation is the restoration ofthe substrate to its initial surface chemistry condition. Using thisrepeated set of steps, a film can be layered onto the substrate in equalmetered layers that are all identical in chemical kinetics, depositionper cycle, composition, and thickness. However, such processes areexpensive and lengthy, requiring vacuum chambers and repeated cycles offilling a chamber with a gas and then removing the gas.

ALD and CVD processes as conventionally taught, typically employ heatedsubstrates on which the materials are deposited. These heated substratesare typically at temperatures above the temperatures organic materialsemployed in OLED devices can tolerate. In addition, the films formed insuch processes may be energetic and very brittle, such that thesubsequent deposition of any materials over the films destroys thefilm's integrity.

Thus, a need exists for an OLED architecture that decreases damage dueto electrode deposition, increases lifetime, and improves the efficiencyof light emission.

SUMMARY OF THE INVENTION

In accordance with one embodiment, the invention is directed towards anOLED device, comprising: a substrate; a first electrode formed over thesubstrate; one or more organic layers formed over the first electrode,at least one organic layer being a light-emitting layer; and a bi-layerelectrode comprising a first transparent conductive protective hermeticlayer formed over the one or more organic layers, and a secondtransparent conductive non-hermetic layer formed over the firsttransparent conductive protective hermetic layer.

In accordance with a further embodiment, the invention is directedtowards a method of forming a top-emitting OLED device, comprising:providing a substrate with a first electrode and one or more organiclayers formed thereon, at least one organic layer being a light-emittinglayer; and forming a bi-layer transparent top electrode by forming afirst transparent conductive protective hermetic layer over the one ormore organic layers at a temperature less than 140 degrees C., andforming a second transparent conductive non-hermetic layer over thefirst transparent conductive protective hermetic layer.

ADVANTAGES

The present invention provides an OLED device having improved yields,lifetime, and light emission efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter of the present invention, itis believed that the invention will be better understood from thefollowing description when taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a cross section of a top-emitting OLED device according to anembodiment of the present invention;

FIG. 2 is a cross section of an OLED device having a scattering layeraccording to an alternative embodiment of the present invention;

FIG. 3 is a cross section of an OLED device having color filtersaccording to an alternative embodiment of the present invention; and

FIG. 4 is a cross section of an OLED device having vias according to analternative embodiment of the present invention.

It will be understood that the figures are not to scale since theindividual layers are too thin and the thickness differences of variouslayers too great to permit depiction to scale.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, an OLED device 8 according to one embodiment of thepresent invention comprises a substrate 10, a first electrode 12 formedover the substrate, one or more organic layers 14 formed over the firstelectrode 12, at least one organic layer being a light-emitting layer,and a bi-layer electrode 16 comprising a first transparent conductiveprotective hermetic layer 17 formed over the one or more organic layers14, and a second transparent conductive non-hermetic layer 18 formedover the first transparent conductive protective layer 17. In aparticular embodiment, first transparent conductive protective layer 17may have a first conductivity, and second transparent conductive layer18 may have a second conductivity greater than the first conductivity.

In a top-emitter embodiment of the present invention, the bi-layerelectrode 16 is a transparent top electrode and the first electrode 12is a bottom electrode. The bottom electrode may be reflective. It ispreferred that the bi-layer electrode 16 has a refractive optical indexequal to or greater than the refractive optical index of the one or moreorganic layers. By providing such relative refractive indices, lightemitted from the organic layers 14 will not be trapped by total internalreflection in the organic layers 14 since light may travel from theorganic layers 14 into the equal- or higher-index bi-layer electrode 16.In various embodiments of the present invention, the first and secondconductive layers 17 and 18 may have approximately the same opticalindex, for example within 0.1, the first and second conductive layers 17and 18 may have an optical index equal to or greater than the opticalindex of the light-emitting layer, and/or the first transparentconductive protective layer 17 may have an optical index equal to orlower than the second transparent conductive layer 18 and equal to orgreater than the optical index of the light-emitting layer 14.

Thin-film electronic components 30 having planarization layers 32 may beemployed to control the OLED device, as is known in the art. A cover 20is provided over the OLED and electrode layers and adhered to thesubstrate 10 to protect the OLED device, for example using an adhesive60. The bottom first electrode 12 is typically patterned to formlight-emitting areas 50, 52, 54. In general, the first and secondconductive layers 17 and 18 are coextensive and are both formed, atleast, over the light-emitting areas 50, 52, 54. One or both of thetransparent conductive layers 17, 18 of the bi-layer electrode 16 may beunpatterned and formed continuously over the organic layers 14.

The first transparent conductive protective layer 17 of the bi-layerelectrode 16 forms a hermetic layer to prevent the passage of moistureand oxygen. Typical prior-art transparent electrodes are sputtered orevaporated.

Applicants have demonstrated that such deposition techniques do not forman adequate barrier layer to the passage of environmental contaminants.A hermetic layer typically forms a barrier having a moisture permeationrate of less than 1×10⁻⁴ and preferably less than 1×10⁻⁵ gm/m²/day andmore preferably less than 5×10⁻⁶ gm/m²/day to adequately protect theOLED materials. Such layers may be formed of ordered and structuredlayers of molecules that completely coat a surface and whose moleculesare in closely adjacent and in alignment with neighboring molecules.Evaporation and sputtering, in contrast, form random arrangements ofmolecules between which other contaminating molecules can pass.

Typically, the first and second conductive layers may be metal oxidelayers or doped metal oxide layers, for example aluminum zinc oxide andindium tin oxide. The first transparent conductive protective layer 17is formed between the one or more organic layers 14 and the secondtransparent conductive layer 18. The first transparent conductiveprotective layer 17 may be relatively thin layer, for example it mayhave a thickness of less than 100 nm, 75 nm, or 50 nm. Applicants havedemonstrated that thick hermetic layers can be brittle and prone tocracking. Hence, a relatively thin layer is preferred. However, thintransparent conductive layers may not provide the desired conductivity.Employing a relatively thin hermetic first conductive layer 17 incombination with a relatively thick, non-hermetic second conductivelayer 18 in accordance with a particular embodiment of the invention mayprovide the combination of reduced brittleness, resistance toenvironmental contamination, and desired conductivity. The secondtransparent conductive layer 18 may be deposited by evaporation orsputtering in accordance with conventional transparent conductive layerdeposition techniques. The first transparent conductive protectivehermetic layer 17 further provides physical protection to the underlyingorganic layers 14 during deposition of the second transparent conductivelayer 18.

A useful transparent electrode for a top-emitting OLED device typicallyhas a high conductivity and provides a good seal, together with hightransparency. The bi-layer electrode 16 of the present invention mayprovide such advantages. Applicants have discovered throughexperimentation that electrodes formed of sputtered metal oxidestypically have a higher conductivity than those deposited through atomiclayer deposition or chemical vapor deposition, while electrodes formedthrough atomic layer deposition or chemical vapor deposition can providea hermetic seal. Hence, in one embodiment of the present invention, thesecond transparent conductive layer 18 may be formed by sputtering orevaporation while the first transparent conductive protective layer 17may be formed by atomic layer deposition or chemical vapor deposition.Moreover, atomic layer deposition or chemical vapor deposition arerelatively slower processes than sputtering or evaporation. Hence, arelatively thin first transparent conductive protective layer 17 formedby atomic layer deposition or chemical vapor deposition may be usefullycombined with a relatively thicker and more conductive secondtransparent conductive layer 18 formed by sputtering or evaporation.Since it is known that sputtering may damage the organic layers 14, thepresent invention locates the first transparent conductive protectivelayer 17 between the organic layers 14 and the second transparentconductive layer 18. Moreover, the first transparent conductiveprotective layer 17 may have improved charge injection into the organiclayer 14 than the second transparent conductive layer 18 or conventionaltransparent electrodes found in the prior art.

Typical prior-art transparent electrodes are formed of sputtered orevaporated transparent conductive oxides and may have a resistivity, forexample, of 100 ohms per square depending on the actual depositionconditions and thickness. However, according to a further embodiment ofthe present invention, by employing an auxiliary electrode 26, thebi-layer electrode 16 may be less conductive and have a resistivitygreater than 100 ohms per square, greater than 500 ohms per square,greater than 1000 ohms per square, or even as high as 10,000 ohms persquare. The smaller the individual light-emitting areas and the moreextensive the auxiliary electrodes, the higher the resistivity of thebi-layer electrode 16 may acceptably be.

Hence, in a further embodiment of the present invention, the OLED devicemay also comprise a patterned auxiliary electrode 26 in electricalcontact with the bi-layer electrode 16. Generally, the patternedauxiliary electrode 26 may be formed between or under the light emittingareas 50, 52, 54. Such an auxiliary electrode 26 may enhance theconductivity of the bi-layer electrode 16. As shown in FIG. 1, theauxiliary electrode 26 may be formed on a side of the bi-layer electrode16 opposite the one or more organic layers 14. Alternatively, as shownin FIG. 4, the auxiliary electrode 26 may be formed on a side of the oneor more organic layers 14 opposite the bi-layer electrode 16 and may beelectrically connected to the bi-layer electrode 16 through a via 34formed in the one or more organic layers 14.

The auxiliary electrode 26 provides additional current-carryingcapability to the device to improve the distribution of current to thevarious light-emitting areas. The auxiliary electrode 26 may be formedbetween the light-emitting areas (as shown in the figures) and may beopaque and formed, for example of thick metal bus lines. Alternatively,the auxiliary electrode 26 may be formed over or under portions of thetransparent electrode, so long as light-emitted by the light-emittinglayer can pass through at least a complementary portion of thetransparent bi-layer electrode 16. Hence, the auxiliary electrode 26preferably is patterned.

In various embodiments of the present invention, the auxiliary electrode26 may be formed by sputtering or evaporating metals through masks, forexamples as described in U.S. Pat. No. 6,812,637 entitled “OLED Displaywith Auxiliary Electrode” by Cok et al. In alternative embodiments ofthe present invention, the auxiliary electrode 26 may be formed on theside of the one or more organic layers 14 opposite the bi-layerelectrode 16 and may be electrically connected to the bi-layer electrode16 through vias 34 formed in the one or more organic layers 14. Theauxiliary electrode may be formed using conventional photolithographictechniques while the vias may be formed using laser ablation, forexample as described in U.S. Pat. No. 6,995,035 entitled “Method ofMaking a Top-Emitting OLED Device having Improved Power Distribution” byCok et al. Materials employed in forming the auxiliary electrode mayinclude aluminum, silver, magnesium, and alloys thereof.

According to further embodiments of the present invention and as furtherillustrated in FIG. 2, a scattering layer 22 may be formed over thebi-layer electrode 16 opposite the organic layer(s) 14. The scatteringlayer 22 scatters light trapped in the bi-layer electrode 16 and organiclayers 14 by total internal reflection. To maintain the sharpness of apixilated OLED device, a low-index element 24 having a refractive indexlower than the refractive index of the organic layers 14 is formedbetween the bi-layer electrode 16 and the transparent cover 20 as taughtin US 2006/0186802 “OLED Device having Improved Light Output” by Cok etal, which is hereby incorporated in its entirety by reference. Thelow-index element 24 may be a vacuum or gas-filled space, as illustratedin the Figures.

In the prior art, the transparent electrode is typically formed asthickly as possible to enhance the conductivity of the electrode.However, as the transparent electrode becomes thicker, the amount oflight absorbed by the electrode increases. A typical transparentelectrode as suggested by the prior art is 100 nm or more thick. Such athickness of ITO, for example, is not perfectly transparent and absorbsabout 4% of the light traveling through the electrode, dependingsomewhat on the frequency of the light and the deposition conditions.This is not a large absorption, so that prior-art designs tend to preferincreased thickness, conductivity, and absorption. However, applicantshave surprisingly found through optical modeling that the impact of theabsorption of light in the transparent electrode is much greater. Lighttrapped within the bi-layer electrode 16 and organic layers 14 willtravel at a higher angle with respect to the normal, therebyencountering a greater thickness of transparent material and resultingin a greater absorption than anticipated. Moreover, the trapped lightwill encounter the transparent electrode multiple times, therebyincreasing the absorption experienced. When a scattering layer isemployed to extract the trapped light, modeling indicates that anaverage light ray will pass through the transparent electrode threetimes at relatively higher angles to the normal, thereby increasing thepractical absorption of the transparent layer by more than a factor ofthree, to greater than 12%. Moreover, an additional encapsulation layerformed over the transparent electrode, as taught by the prior art, mayfurther increase the amount of light trapped and absorbed. Hence, byemploying a hermetic, reduced-thickness transparent conductiveprotective layer 17 without the use of an additional encapsulationlayer, additional light may be extracted from the OLED device and therequisite conductivity provided by employing an auxiliary electrode.

Applicants have demonstrated that scattering layers provided directly onthe transparent electrode have improved light extraction over designsthat employ an additional encapsulation layer over the electrode.However, when performed over a conventional sputter-depositedtransparent ITO electrode, the scattering layer deposition process(typically incorporating use of organic solvents) causes dark spots togrow, reducing device lifetime. The presence of a bi-layer electrodewith a first conductive layer formed using the vapor deposition processprovides a barrier to the scattering layer solvent, and does not causedark spots to grow, indicating that an improved hermetic layer ispresent.

In some embodiments of the present invention (FIG. 3), thelight-emitting organic layer 14 may emit white light, in which casecolor filters 40R, 40G, 40B may be formed, for example on the cover 20,to filter light to provide a full-color light-emissive device havingcolored light-emitting elements 50, 52, 54.

As employed herein, a hermetic layer is one in which atomic layer orvarious chemical vapor deposition processes are employed to form a layerresistive to penetration by moisture and oxygen. The hermetic firsttransparent conductive protective layer 17 may be formed by employing avapor deposition process comprising alternately providing a firstreactive gaseous material and a second reactive gaseous material,wherein the first reactive gaseous material is capable of reacting withthe organic layers treated with the second reactive gaseous material.Alternatively, the first conductive protective layer 17 is formed usingan atomic layer deposition process, a vacuum chemical vapor depositionprocess, or atmospheric chemical vapor deposition process. Theseprocesses are similar in their use of complementary reactive gases,either in a system with a vacuum purge cycle or in an atmosphere.Generally, it is preferred to form the first transparent conductiveprotective layer 17 at a temperature less than 140 degrees C. to avoiddamaging the underlying organic layers. More preferably, the firsttransparent conductive protective layer 17 may be formed at atemperature less than 120 degrees C. or less than 100 degrees C.Applicants have successfully formed hermetic layers over organicmaterials using zinc oxide based compounds. Moreover, effective hermeticlayers have been formed at temperatures of 100 degrees C.

Such layers are formed by alternately providing a first reactive gaseousmaterial and a second reactive gaseous material, wherein the firstreactive gaseous material is capable of reacting with the organic layerstreated with the second reactive gaseous material. The first reactivegaseous material completely covers the exposed surface of the organiclayers 14 while the second reactive gaseous material reacts with thefirst reactive gaseous material to form a layer highly resistant toenvironmental contaminants. In contrast, layers deposited byconventional means such as evaporation or sputtering do not formhermetic layers. Applicants have demonstrated the problems of theconventional deposition art for transparent electrodes in protectingorganic materials and the improvements found by employing a hermeticlayer formed by a vapor deposition process. Moreover, the vapordeposition process itself reduces the damage incurred by the underlyingorganic layers as compared to conventional sputtering processes.

According to the present invention, the first transparent conductiveprotective layer 17 is formed at a temperature less than 140 degrees C.In typical, prior-art atomic layer deposition or chemical vapordeposition processes, the substrate and any layers coated thereon areheated to relatively high temperatures, for example >200 degrees C. Suchhigher temperatures may be useful in increasing the conductivity ofdeposited layers. However, according to the present invention, a reducedtemperature for deposition of the first transparent conductive hermeticlayer employed in combination with a second transparent non-hermeticlayer enable adequate transparent electrode conductivity. In furtherembodiments, an auxiliary electrode may be further employed to provideevent further effective conductivity. In a more preferred embodiment ofthe present invention, the first transparent conductive protective layer17 is formed at a temperature less than or equal to 120 degrees C., lessthan or equal to 100 degrees C., or less than or equal to 80 degrees C.

A wide variety of metal oxides may be employed to form the conductivelayers 17 and 18. In preferred embodiments, the zinc oxide, aluminumzinc oxide, and/or indium tin oxide, may be employed. In general,dopants may be employed to improve the conductivity of the electrode,for example by employing aluminum with zinc oxide to form aluminum zincoxide. While the first transparent conductive hermetic layer may have aconductivity less than that of the second transparent conductive layer,it is preferred that each of the first and second transparent conductiveprotective layer have a resistivity of less than 10,000 Ohm/sq.

A variety of thicknesses may be employed for the bi-layer electrode 16,depending on the subsequent processing of the device and environmentalexposure. The thickness of the first transparent conductive protectivelayer 17 may be selected by controlling the number of sequentiallydeposited layers of reactive gases. In one embodiment of the presentinvention, the first conductive protective layer 17 may be less than 100nm thick, less than 75 nm thick, or less than 50 nm thick. Typically,the first conductive protective layer 17 will be thinner than the secondtransparent conductive layer 18.

For top-emitting OLED devices, substrate 10 may be opaque to the lightemitted by OLED device 8. Common materials for substrate 10 are glass orplastic. First electrode 12 may be reflective. Common materials forfirst electrode 12 are aluminum and silver or alloys of aluminum andsilver. Organic EL element 14 includes at least a light emitting layer(LEL) but frequently also includes other functional layers such as anelectron transport layer (ETL), a hole transport layer (HTL), anelectron blocking layer (EBL), or a hole blocking layer (HBL), etc. Thepresent invention is independent of the number of functioning layers andindependent of the materials selection for the organic EL element 14.Often a hole-injection layer is added between organic EL element 14 andthe anode and often an electron injection layer is added between organicEL element 14 and the cathode. In operation a positive electricalpotential is applied to anode and a negative potential is applied to thecathode. Electrons are injected from the cathode into organic EL element14 and driven by the applied electrical field to move toward the anode;holes are injected from the anode into organic EL element 14 and drivenby the applied electrical field to move toward the cathode. Whenelectrons and holes combine in organic EL element 14, light is generatedand emitted by OLED device 8.

Material for the bi-layer electrode 16 can include inorganic oxides suchas indium oxide, gallium oxide, zinc oxide, tin oxide, molybdenum oxide,vanadium oxide, antimony oxide, bismuth oxide, rhenium oxide, tantalumoxide, tungsten oxide, niobium oxide, or nickel oxide. These oxides areelectrically conductive because of non-stoichiometry. The resistivity ofthese materials depends on the degree of non-stoichiometry and mobility.These properties as well as optical transparency can be controlled bychanging deposition conditions. The range of achievable resistivity andoptical transparency can be further extended by impurity doping. Evenlarger range of properties can be obtained by mixing two or more ofthese oxides. For example, mixtures of indium oxide and tin oxide,indium oxide and zinc oxide, zinc oxide and tin oxide, or cadmium oxideand tin oxide have been the most commonly used transparent conductors.

A top-emitting OLED device of the present invention may be formed byproviding a substrate with a bottom electrode and one or more organiclayers formed thereon, at least one organic layer being a light-emittinglayer, and forming a bi-layer transparent top electrode by forming afirst transparent conductive protective hermetic layer over the one ormore organic layers at a temperature less than 140 degrees C., andforming a second transparent conductive non-hermetic layer over thefirst transparent conductive protective hermetic layer.

According to various embodiments of the present invention, the firsttransparent conductive protective layer 17 may be deposited by vapordeposition. As used herein, vapor deposition refers to any depositionmethod that deposits a first reactive material onto a substrate. Asubsequent second reactive material is then provided to react with thefirst reactive material. The process is repeated until an adequatemulti-layer thickness is formed. For the description that follows, theterm “gas” or “gaseous material” is used in a broad sense to encompassany of a range of vaporized or gaseous elements, compounds, ormaterials. Other terms used herein, such as: reactant, precursor,vacuum, and inert gas, for example, all have their conventional meaningsas would be well understood by those skilled in the materials depositionart.

The first transparent conductive protective layer 17 may be formed usingan atomic layer deposition process, a vacuum chemical vapor depositionprocess, or atmospheric chemical vapor deposition process and may beformed at a temperature less than 140 degrees C., less than 120 degreesC., or less than 100 degrees C.

While prior-art atomic layer deposition processes may be employed, inone embodiment of the present invention, a moving, gas distributionmanifold having a plurality of openings through which first and secondreactive gases are pumped is translated over a substrate to form a firsttransparent conductive, protective layer 17. Co-pending, commonlyassigned U.S. Ser. No. 11/392,007, filed Mar. 29, 2006, describes such amethod in detail and the disclosure of which is hereby incorporated inits entirety by reference. However, the present invention may beemployed with any of a variety of prior-art vapor deposition methods.

The first transparent conductive protective layer deposition process mayemploy a continuous (as opposed to pulsed) gaseous materialdistribution. The conductive protective layer deposition process citedabove allows operation at atmospheric or near-atmospheric pressures aswell as under vacuum and is capable of operating in an unsealed oropen-air environment. Preferably, the protective layer depositionprocess proceeds at an internal pressure greater than 1/1000 atmosphere.More preferably, the transparent protective layer is formed at aninternal pressure equal to or greater than one atmosphere. Various gasesmay be employed, including inert gases such as argon, air, or nitrogen.In any case, it is preferred that the gas be dry to avoid contaminatingthe organic materials with moisture.

A continuous supply of gaseous materials for the system may be providedfor depositing a thin film of material on a substrate. A first molecularprecursor or reactive gaseous material may be directed over thesubstrate and reacts therewith. In a next step, a flow with inert gasoccurs over the area. Then, in one embodiment of the present invention,relative movement of the substrate and the distribution manifold mayoccur so that a second reactive gas from a second orifice in adistribution manifold may react with the first reactive gas deposited onthe substrate. Alternatively, the first reactive gas may be removed fromthe deposition chamber and the second reactive gas provided in thechamber to react with the previous layer on the substrate to produce(theoretically) a monolayer of a desired material. Often in suchprocesses, a first molecular precursor is a metal-containing compound ingas form, and the material deposited is a metal-containing compound, forexample, an organometallic compound such as diethylzinc. In such anembodiment, the second molecular precursor can be, for example, anon-metallic oxidizing compound. Inert gases may be employed between thereactive gases to further ensure that gas contamination does not occur.The cycle is repeated as many times as is necessary to establish adesired film.

The primary purpose of the second molecular precursor is to conditionthe substrate surface back toward reactivity with the first molecularprecursor. The second molecular precursor also provides material fromthe molecular gas to combine with metal at the surface, formingcompounds such as an oxide, nitride, sulfide, etc, with the freshlydeposited metal-containing precursor.

According to the present invention, it may not be necessary to use avacuum purge to remove a molecular precursor after applying it to thesubstrate. Purge steps are expected by most researchers to be the mostsignificant throughput-limiting step in ALD processes.

Assuming that, for example, two reactant gases AX and BY are used. Whenthe reaction gas AX flow is supplied and flowed over a given substratearea, atoms of the reaction gas AX may be chemically adsorbed on asubstrate, resulting in a layer of A and a surface of ligand X(associative chemisorptions). Then, the remaining reaction gas AX may bepurged with an inert gas. Then, the flow of reaction gas BY, and achemical reaction between AX (surface) and BY (gas) occurs, resulting ina molecular layer of AB on the substrate (dissociative chemisorptions).The remaining gas BY and by-products of the reaction are purged. Thethickness of the thin film may be increased by repeating the processcycle many times.

Because the film can be deposited one monolayer at a time it tends to beconformal and have uniform thickness and will therefore tend to fill inall areas on the substrate, in particular in pinhole areas that mayotherwise form shorts. Applicants have successfully demonstrated thedeposition of a variety of thin-films, including zinc oxide films overorganic layers. The films can vary in thickness, but films have beensuccessfully grown at temperatures of 100 degrees C. and of thicknessesranging from a few nanometers to 100 nm. Preferably, the films arestructured such that moisture permeability is minimized, for examplewith more crystalline films. To provide desired conductivity, in apreferred embodiment doped metal oxide compositions may be deposited,e.g., ZnO:Al, ZnS:Mn, SrS:Ce, Al₂O₃:Er, ZrO₂:Y and the like.

Various gaseous materials that may be reacted are also described inHandbook of Thin Film Process Technology, Vol. 1, edited by Glocker andShah, Institute of Physics (IOP) Publishing, Philadelphia 1995, pagesB1.5:1 to B1.5:16, hereby incorporated by reference; and Handbook ofThin Film Materials, edited by Nalwa, Vol. 1, pages 103 to 159, herebyincorporated by reference. In Table V1.5.1 of the former reference,reactants for various ALD processes are listed, including a firstmetal-containing precursors of Group II, III, IV, V, VI and others. Inthe latter reference, Table IV lists precursor combinations used invarious ALD thin-film processes.

Optionally, the present first transparent conductive layer depositionprocess can be accomplished with the apparatus and system described inmore detail in commonly assigned, copending U.S. Ser. No. 11/392,006,filed Mar. 29, 2006 by Levy et al. and entitled, “APPARATUS FOR ATOMICLAYER DEPOSITION”, hereby incorporated by reference.

In a preferred embodiment, ALD can be performed at or near atmosphericpressure and over a broad range of ambient and substrate temperatures.Within the context of the present invention, however, temperatures equalto or less than 140° C. are required to avoid damage to organic layers.Preferably, a relatively clean environment is needed to minimize thelikelihood of contamination; however, full “clean room” conditions or aninert gas-filled enclosure would not be required for obtaining goodperformance when using preferred embodiments of the process of thepresent invention.

OLED devices of this invention can employ various well-known opticaleffects in order to enhance their properties if desired. This includesoptimizing layer thicknesses to yield maximum light transmission,providing dielectric mirror structures, replacing reflective electrodeswith light-absorbing electrodes, providing anti-glare or anti-reflectioncoatings over the display, providing a polarizing medium over thedisplay, or providing colored, neutral density, or color conversionfilters over the display. Filters, polarizers, and anti-glare oranti-reflection coatings may be specifically provided over the cover oras part of the cover.

The present invention may also be practiced with either active- orpassive-matrix OLED devices. It may also be employed in display devicesor in area illumination devices. In a preferred embodiment, the presentinvention is employed in a flat-panel OLED device composed ofsmall-molecule or polymeric OLEDs as disclosed in but not limited toU.S. Pat. No. 4,769,292, issued Sep. 6, 1988 to Tang et al., and U.S.Pat. No. 5,061,569, issued Oct. 29, 1991 to VanSlyke et al. Manycombinations and variations of organic light-emitting displays can beused to fabricate such a device, including both active- andpassive-matrix OLED displays having either a top- or bottom-emitterarchitecture.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

PARTS LIST

8 OLED device

10 substrate

12 electrode

14 organic element layer

16 bi-layer electrode

17 first transparent conductive protective hermetic layer

18 second transparent conductive non-hermetic layer

20 cover

22 scattering layer

24 low-index element

26 auxiliary electrode

30 thin-film electronic components

32 planarization layers

34 via

40R, 40G, 40B color filters

50 light-emitting area

52 light-emitting area

54 light-emitting area

60 adhesive

1. An OLED device, comprising: a substrate; a first electrode formedover the substrate; one or more organic layers formed over the firstelectrode, at least one organic layer being a light-emitting layer; anda bi-layer electrode comprising a first transparent conductiveprotective hermetic layer formed over the one or more organic layers,and a second transparent conductive non-hermetic layer formed over thefirst transparent conductive protective hermetic layer.
 2. The OLEDdevice of claim 1, wherein the first and second conductive layers aremetal oxide layers.
 3. The OLED device of claim 1, wherein the firsttransparent conductive protective layer has thickness of less than 100nm.
 4. The OLED device of claim 1, wherein the first transparentconductive protective layer has thickness of less than 75 nm.
 5. TheOLED device of claim 1, wherein the second transparent conductive layeris formed by sputtering or evaporation.
 6. The OLED device of claim 1,wherein the first transparent conductive protective layer is formed byatomic layer deposition or chemical vapor deposition.
 7. The OLED deviceof claim 1, further comprising a patterned auxiliary electrode inelectrical contact with the bi-layer electrode.
 8. The OLED device ofclaim 1, wherein first transparent conductive protective layer has afirst conductivity and the second transparent conductive layer has asecond conductivity greater than the first conductivity.
 9. The OLEDdevice of claim 1, wherein first transparent conductive protective layerhas a resistivity of less than 10,000 Ohm/sq.
 10. The OLED device ofclaim 1, wherein the first and second conductive layers haveapproximately the same optical index.
 11. The OLED device of claim 1,wherein the first and second conductive layers have an optical indexequal to or greater than the optical index of the light-emitting layer.12. The OLED device of claim 1, wherein the first transparent conductiveprotective layer has an optical index equal to or lower than the secondconductive layer and equal to or greater than the optical index of thelight-emitting layer.
 13. The OLED device of claim 1, further comprisinga low-index element formed on a side of the bi-layer electrode oppositethe one or more organic layers, having an optical index less than theoptical index of the light-emitting layer.
 14. The OLED device of claim1, further comprising a scattering layer formed on the bi-layerelectrode.
 15. The OLED device of claim 1, wherein the first transparentconductive protective layer comprises zinc oxide or doped zinc oxide, oraluminum zinc oxide.
 16. The OLED device of claim 1, wherein the secondtransparent conductive layer comprises indium tin oxide.
 17. A method offorming a top-emitting OLED device, comprising: providing a substratewith a first electrode and one or more organic layers formed thereon, atleast one organic layer being a light-emitting layer; and forming abi-layer transparent top electrode by forming a first transparentconductive protective hermetic layer over the one or more organic layersat a temperature less than 140 degrees C., and forming a secondtransparent conductive non-hermetic layer over the first transparentconductive protective hermetic layer.
 18. The method of claim 17,wherein the first transparent conductive protective layer is formed byemploying a vapor deposition process comprising alternately providing afirst reactive gaseous material and a second reactive gaseous material,wherein the first reactive gaseous material is capable of reacting withthe organic layers treated with the second reactive gaseous material.19. The method of claim 17, wherein the first transparent conductiveprotective layer is formed using an atomic layer deposition process, avacuum chemical vapor deposition process, or atmospheric chemical vapordeposition process.
 20. The method of claim 17, wherein the secondtransparent conductive layer is formed by sputter deposition.